I've compared AXY and Fatboy in my Spoiler (thanks to fellow-member 0vid). On the inside, the AXY is completely identical to the MXY, it just has a larger shell (same size as a single-coil or a Fatboy).

The Fatboy definitely adds "oomph", but also loses a lot of brightness, compared to AXY/MXY. I loved the oomph, but couldn't live with the lack of brightness. However, I can imagine it will be wonderful on a fretless.